Where teeth have been lost, as a result of, for example, dental disease, it may become necessary to install dental implants. Dental implants are usually made of titanium or other bio-compatible materials, and form a strong inflexible union with bone. This type of union has been termed "osseointegration". It is now understood that osseointegration will endure for many years in favorable circumstances. However this union may rapidly break down if the implant is overloaded by unfavorable forces.
It is common practice to use several implants to support a dental bridge. It is also common practice to use retaining screws to connect such bridges to the implants.
Bridges are presently fabricated by casting wax or plastic patterns in dental alloys, or by using computer aided design and computer aided milling technology. Most stages of construction are carried out on models prepared from impressions of the patient's mouth. There is thus a great potential for error in the fabrication of the bridge framework, and so it is not surprising that small discrepancies in the mechanical fit of the bridge superstructure to the dental implants arise.
When positioned in the mouth a small discrepancy may sometimes go unnoticed with the result that, when the bridge is screwed into place, in spite of a small error in fit, flexure of the implant, surrounding bone, bridge superstructure, retaining screen, and any other prosthetic components may permit the bridge to be seated in spite of the imperfect fit. As a result of this flexure, there may then be a considerable build up of stress or strain in the system. This later manifests itself, as failure of one of the components, or failure of osseointegration and ultimately the loss of the implant.
When the implant supported bridge superstructure fits against the implants precisely it is commonly said to fit "passively" that is, when the screws are tightened no stress or strain is introduced. Under these circumstances, tightening the screws that retain the bridge superstructure merely compresses the framework against the head of the implant.
The rapid increase in torque as the retaining screws arc tightened when the screw head contacts the bridge superstructure, and the superstructure fits the implants passively is illustrated in FIG. 1.
Where the implant retained bridge superstructure fits the implants poorly, tightening the retaining screws results in a relatively slower build up of tension in the screw as the various components of the system are flexed.
The tension in (or torque applied to) the retaining screw increasing slowly as the various components of the system are flexed, and then rapidly as the superstructure is compressed against the implant when the superstructure finally contacts the implant is shown in FIG. 2.